TREM2 and Alzheimer disease: Although rare in the population, certain TREM2 variants are associated with increased risk of developing neurodegenerative diseases such as Alzheimer’s disease (AD) [21, 26], frontotemporal dementia [20, 56], Parkinson’s disease [56], and amyotrophic lateral sclerosis [7], presumably as a consequence of TREM2 loss of function [12].